Method of making a magnetic recording head

ABSTRACT

A method for making the core of a magnetic recording or playback head that comprises bending a flat strip of core material into an intermediate product, a W-form, i.e., having at its outer or remote ends two vertical legs, which join, at the bottom, two bottom segments which extend straight toward one another, and which eventually form the sides of the core. These bottom segments join two inclined segments which rise angularly, like an inverted V, to an apex. This apex and the tops of the two end legs are then simultaneously lap ground in a common plane, until the &#39;&#39;&#39;&#39;apex&#39;&#39;&#39;&#39; is almost ground away, leaving an extremely thin amount of connecting material between the inclined segments. This bit of remaining connecting material forms an easily bendable hinge between the two inclined segments. The two halves of this intermediate product are then bent through 180* on this hinge until the two ground ends of the first mentioned legs abut to form a so-called back gap. The two core ends on opposite sides of the hinge are then closed and grip between them a thin, nonmagnetic spacer or shim, of gap thickness. The core in that form is encapsulated in a metal case and potted therein with polymerized resins. The hinge, when in this position, is located exterior to the gap, and forms a projection through an aperture in the case. By grinding operation, the hinge, a portion of the core just under it and of the top wall of the case, are ground away, leaving the gap exposed. In a modification, the core is composed of laminations, stamped initially to the shape of the intermediate product. These are assembled, and the assembly then treated as in the first procedure. In still another modification, the core and a holder are first assembled, and it is the holder rather than initial part of core that forms the hinge.

United States Patent 1 Lipps 1 1 Oct. 15, 1974 METHOD OF MAKING AMAGNETIC RECORDING HEAD [76] Inventor: Edwin Alvin Lipps, 205

Chautauqua, Pacific Palisades, Calif. 90272 22 Filed: 0ct.16,1l973 21App1.No.:406,8711

[52] US. Cl. 29/603, 29/418, 179/1002 C [51] int. Cl. Gllb 5/42 [58]Field of Search 29/603, 418; 179/1002 C;

340/l74.l F; 346/74 MC Primary ExaminerCharles W. Lanham AssistantExaminerCarl E. Hall Attorney, Agent, or Firm-Forrest .1. Lilly [5 7ABSTRACT A method for making the core of a magnetic recording orplayback head that comprises bending a flat strip of core material intoan intermediate product, a W-form, i.e., having at its outer or remoteends two vertical legs, which join, at the bottom, two bottom segmentswhich extend straight toward one another, and which eventually form thesides of the core. These bottom segments join two inclined segmentswhich rise angularly, like an inverted V, to an apex. This apex and thetops of the two end legs are then simultaneously lap ground in a commonplane, until the apex is almost ground away, leaving an extremely thinamount of connecting material between the inclined segments. This bit ofremaining connecting material forms an easily bendable hinge between thetwo inclined segments. The two halves of this intermediate product arethen bent through 180 on this hinge until the two ground ends of thefirst mentioned legs abut to form a so-called back gap. The two coreends on opposite sides of the hinge are then closed and grip betweenthem a thin, non-magnetic spacer or shim, of gap thickness. The core inthat form is encapsulated in a metal case and potted therein withpolymerized resins. The hinge, when in this position, is locatedexterior to the gap, and forms a projection through an aperture in thecase. By grinding operation, the hinge, a portion of the core just underit and of the top wall of the case, are ground away, leaving the gapexposed. In a modification, the core is composed of laminations, stampedinitially to the shape of the intermediate product. These are assembled,and the assembly then treated as in the first procedure.

In still another modification, the core and a holder are firstassembled, and it is the holder rather than initial part of core thatforms the hinge.

16 Claims, 18 Drawing Figures PAIENIED B 1 51914 8. 840. 984

sum 2 or 2 E m'r'" METHOD OF MAKING A MAGNETIC RECORDING HEAD FIELD OFTHE INVENTION This invention relates generally to magnetic tape heads,i.e., electromagnetic heads for recording information on magnetic media,such as a tape, or reproducing or erasing it therefrom.

BACKGROUND OF THE INVENTION The critical magnetic iron core parts ofsuch heads are manufactured in two C-like core halves. The ends of theseC-cores are lapped to virtually perfect flatness. They are thenassembled to the coil with a spacer to size the gap length, and are thenencapsulated with epoxy resins to mechanically fix the assembly withinthe case. This operation is performed visually and requires greatoperational skill. As with all such highly skilled operations, there arenumerous rejects due to operator error. The only alternative heretoforehas been the use of highly precise and very expensive tools and jigs.

The general object of the invention is to provide a relatively simplyexecuted and inexpensive process for making cores for theseelectromagnetic heads, and which results in a superior, dimensionallyprecise core gap, yielding improved translation efficiency, and uniformfrequency response along the gap.

BRIEF DESCRIPTION OF THE INVENTION The invention involves, in a presentpreferred embodiment, the forming of the core from a strip of fiatmagnetic core material by bending it initially into a W- form. Theintermediate apex and the two outside legs of the W-form are then lapground, preferably in a common plane, until the core thickness at theapex is, typically, 1 to mils. Thus, a thin ductile hinge is formed. Thetwo halves of the W-forrn are then bent towards one another on thishinge through an angle of 180, until the aforementioned outside legsabut one another, at a so-called back gap, a coil being first placedaround one or both of the core halves. On opposite sides of the hinge,the core ends approach one another during this bending operation. Thegap filler, in the form of a very thin non-magnetic shim, plate or chip,e.g., is inserted and finally pinched between the closing core ends.Theback gap is squeezed together by some means such, e.g., as a springclip. The assembly is now inserted into a case for encapsulation withpolymerizing resins. Afterthe resins are cured, the head face is groundto size. The external hinge, which has been contiguous to the corethroughout the operation, is now ground away, exposing the gap in itsnormal configuration.

The hinge, until ground away, preserves the gap dimensionally andgeometrically, a result never, to my knowledge, heretofore accomplished.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a side elevational view of acore bent into its W-form;

FIG. 2 is a side elevational view of the core of FIG. 1, after lapgrinding;

FIG. 3 is a view of the core form of FIG. 2, but showing the two halvesthereof bent part way toward final form;

FIG. 3a is an enlarged detail of a portion of FIG. 3;

FIG. 4 is an exploded perspective view of the core form approximately inthe position of FIG. 3, but showing a coil placed on one leg thereof,and showing a spring clip used for holding the back gap in the coreclosed, and also the non-magnetic gap spacer in position to be insertedinto the still wide-open gap space;

FIG. 4a is a detail showing the core bent nearly closed, and a gapfiller in the pocket;

FIG. 5 is a longitudinal medial sectional view of the magnetic head,showing the core and coil bent to their final position, assembled withterminals, and inserted inside the case;

FIG. 6 is an enlarged view of the upper portion FIG. 7, showing theepoxy resin filler material within the case, and showing also the upperportion of the case and the gap hinge ground away to finalconfiguration;

FIG. 7 is a side elevational view of the W-form core in a presentsuccessfully used manufacturing design;

FIG. 8 shows the upper portion of the head of FIG. 7, with the core bentto its final position, and the core and case ground to their finalcontour;

FIG. 9 is an enlarged detail of a modification of FIG.

FIG. 10 shows an enlarged detail of a modification of FIG. 2;

FIG. 11 shows the hinge and gap portion of the core of FIG. 10 bent to aposition in which the dimple is closed;

FIG. 11a shows the hinge and gap portions of the core of FIG. 10 in afinally closed position;

FIGS. 12 and 13 show two successive positions of a laminated core; and

FIGS. 14 and 15 show two successive positions of another modification.

DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT A strip 10 of magnetic ironalloy core material, Permalloy, i.e., Mu metal, of standard crosssectional dimensions, is bent, forexample, in a bending press, or apunch press containing the properly shaped dies, into an intermediateproduct 11, as shown in FIG. 1, which is sometimes hereinafter referredto (because it resembles the letter W) as a W-form. This intermediateproduct is characterized by two angularly related segments 12,converging asan inverted letter V, to an apex 13. The lower spaced-apartends of the segments 12 merge with horizontal, oppositely extending sidesegments 12', which merge, in turn, at their remote ends, with verticallegs 14. The legs 14 preferably initially rise to, or preferablyslightly above, a horizontal reference plane H that contacts the apex13. They may initially rise above the plane H, or may not quite reachit; but they should, in the illustrative case, rise at least slightlyabove the level to which the apex will presently be ground down, as tobe explained hereinafter.

A second plane of reference C is vertical and bisects the geometriccenter. of thew-form, and thus bisects the apex 13. The directionshorizontal and vertical as used herein have no meaning, of course,beyond the establishment of a convenient frame of reference.

The apex 13 maybe somewhat rounded at the top, as at 18, with aninterior angle underneath, which will generally be relieved or have arounded fillet 19. The exteriorly rounded top surface 18 will be seen topoint or face generally in the direction of convergence of the segments16. Thus, in other language, the apex points in parallelism with, i.e.,in the same direction as, the two legs 14.

The core is then annealed to relieve any work hardening or magneticdamage caused by the stamping and- /or forming operation.

The upper ends of the legs 14, and the apex 13, are then simultaneouslylap ground in a common plane using generally and preferably a singlelap, down to a lap limit line 21 which is located a distance of theorder of l mil to 5 mils above the underneath formation or fillet 19between the convergent segments 12 leading to the apex. See FIG. 2. Forsome special case, this grinding may not be done simultaneously, at bothleg ends and the apex, but in general is an important feature ofadvantage of the invention. Thus the apex 13 is modified to haveprecisely flat lap ground upper surface 15, bisected by the verticalreference plane C. Thus a bit of remaining core metal, a few mils inthickness, is left to join the two halves 13a and 13b of the groundsurface 15 at the center plane C, to function, by bending, as theaforementioned hinge, hereinafter designated by reference numeral 20.The core material, being annealed, has low elasticity and is veryductile, so that it bends easily as a hinge, and has low spring-back.The two halves of the W-form may be bent on this hinge (see FIG. 3) anumber of times before the hinge will fatigue and break. Ordinarily,however, only one bend is required. I

While the core magnet coil may be wound or placed on one of the corelegs 14 with the core still in the W- form of FIG. 2, or a coil might bewound on each of the sides 13 with the core still in the U-form, thecoil is generally assembled with the core bent about to the form of FIG.3. Thus, a wire coil 22, wound on a plastic bobbin 24, with arectangular core hole 25 through it, is

slipped onto one leg 14 thereof (see FIG. 4). A thin,

flat non-magnetic filler or shim 26 is provided (FIGS. 4 and 4a) to gointo the pocket or angle between the two flat core faces above thehinge. The two halves of the initial W-form are then further bent on thehinge 20 until the legs 14 abut one another, thus forming the so calledback gap 27. A metal U-shaped spring clip 28 is slipped on over thesides 12 of the core, and holds the back gap closed.

The plastic bobbin flange 30 at each end of the coil mounts a conductiveterminal strip 31, and the two upper ends of these terminal strips willbe understood to be connected to opposite ends of the coil, while theirlower ends project below the open bottom metal case 33 into which thecore assembly as described is inserted. j

Attention is directed to the fact that the core metal at the hinge 20has, byreason of the bending operation by which the W-form is changed tothe shape shown in FIG. 5, been subjected to tension, in the region atand just above the surface of the fillet 19, and thereby stretched andforced outwardly from between the core faces 13a and 13b to produceexterior ridge or loop 29. This automatic rise or extrusion of the hinge20 into this exterior ridge or loop facilitates its removal in asubsequent step as the gap is completed. Further advantages of thisfeature will be stressed hereinafter.

A metal case 33 receives the core and coil assembly with a close fit,and the hinge ridge 29 protrudes upwardly through a rectangular aperture34 in a rounded top wall 35 of this case. With the core. and coilassembly in position in the case (FIG. 5), it is permanentlyencapsulated therein with polymerizing resins, according to standardpractice. This potting material is indicated in FIG. 6 at 36.

The head face of the case and the protruding hinge ridge are next groundto final configuration along the slightly curved grind line 37 (FIGS. 5and 6). In this operation, the top wall 35 of the case is ground downsomewhat, and the entirety of the exterior hinge ridge 29 is removed.The outline of the parts before this grinding operation is shown inphantom lines at 39 in FIG. 6. The grinding operation removes the hingeridge 20 entirely and may remove some of the core and nonmagnetic filler26 at the gap. It also thins the top wall of the case, as indicated. Thetop of the head is thus defined by a smooth, slightly convex curve, withthe tops of the two core segments exposed and spaced by the gap fillershim 26. v i

It will now be evident that the dimensions of the gap are positivelyfixed against geometrical distortions so long as the hinge 20 remainsintact, so that nonparallelism, i.e., a V-gap, a step or offset gap, anopen gap, and all other conceivable distortions, are prevented.

The hinge is not removed until after the core assembly has been fixed inthe polymerizing resins in the case. The precise gap dimensions are thuspermanently established. The removal of the hinge has also theincidental but important advantage that all material magneticallydamaged by the bending operation is removed from the core. A furthernoteworthy feature of the invention 'is that, in general, all four gapsurfaces are lapped simultaneously in a common plane, thus assuringuniformity and alignment of gap surfaces. For any special exceptions tothis rule, the invention still preserves the hinge intact until the gapis fixed, thus assuring this dimensional control. FIG. 6 shows the gapregion of the ultimate product. The connecting hinge 20 has beenremoved, and the final magnetic gap is filled by the non-magnetic fillershim 26.

In FIGS. 7 and 8 I have shown, to scale, a present successfulmanufacturing design of the head. Parts of the head in these figurescorresponding to parts of FIGS. 1-6 are identified by the same referencenumerals, and only the modifications will be described. FIG. 7corresponds to FIG. 1 in that it shows the initial core strip just afterforming to its initial W-form. The core in this instance is fabricatedfrom a strip of 0.022 inch thick Mu metal. The apex 13 of the angularsegments 12 is formed on curves of radii substantially increased overthose at 18 and 19 in FIG. 1, as will be evident. The inclined segments12 of the core form comprise in this case two lower portions 42 and 43,inclined at an inclined angle of about 54, and two upper portions 44 and45, respectively, which are offset towards one another, and steepened sothat each is at about 7 to vertical. The top of the apex is thus in theshape of an arch 46 in this design, facilitating manufacture.

The second step is again the lap grinding operation, which in this casegrinds the two legs 14 and the arch at the top of the modif ed apex downto the lap line 47, which is located a distance of the order of 1 milfrom the high point of the underside of the arch 46. The

bend axis then falls above the center of what remains of the arch 46.

The upper portion of the lapped W-form is shown in FIG. 8, after bendingin the manner of the earlier embodiment, with an upper portion of thecase also shown. The figure shows the case and core after grinding, theground-away parts being indicated in phantom lines.

FIG. 9 shows, to an enlarged scale, a modification of the embodiment ofFIGS. 7 and 8. Here the underside arch of the apex is formed with aninverted V-notch 49 to locate accurately the point where the hingeaction will occur. The distance from the bottom of the notch to the topside of the arch is of the order of a mil. This modification is usefulwhen a wide arched apex is employed.

Returning now to the embodiment of FIGS. 1-6, a refinement of importancewill be described. As the core halves are bent on their hinge to theform of FIGS. 3 and 4, and then especially as they close to the positionof FIG. 5, there is, as earlier described, a tension in the hinge regionbetween the two hinge connected segments 12 that cause a stretching andextrusion of the hinge from between the two closing halves 13a and 13bof the ground upper surface 15. It is desirable that this hinge materialbe extruded as much as possible from between these closing core faces13a and 13b. As the two core halves are bent on the hinge, the thinlayer of core metal that constitutes the hinge 20 turns or fulcrumsabout a neutral axis n located about one third of the way down from theupper to the lower surfaces of the hinge. As viewed in FIGS. 3 and3a,the hinge material above this axis is in compression, and the hingematerial below the axis is under tension, and stretches. The stretchedmaterial bows outwardly, finally forming an extruded externally locatedhinge ridge or loop. However, the operation tends to compress a smallamount of the hinge material above the axis, and leave it as a smallbead of metal between the surfaces 13a and 1312.

I have found it possible to extrude the entirety of this hinge metal, sothe faces 113a and 13b will close perfectly flush on one another. Thiscan be done, with the core form still in the position of FIG. 2, byforming a wedge-shaped indentation or dimple 48 in and across the verycenter of the ground face 15 (see FIG. 10). This can be done with adimple die. This dimple has a preferred depth of about'one third thethickness of the remaining core metal at the hinge. A correspondingslight bulge 46 may occur at the bottom of the hinge. The hinge actionthen takes place, at the beginning, about a new neutral axis spaced alittle below the bottom of this dimple. When the core form is thenturned further on the dimpled hinge, the faces 13a and 13b rotate orfulcrum on the new, slightly lowered axis, until the opposite faces ofthe wedge shaped dimple have closed on one another. (FIG. 11). The twohalves of the core then fulcrum on the line d of the closed dimple (FIG.11), with no part of the hinge region thereafter in compression.Instead, the hinge region is now all in tension, and stretches more andmore as the bending action is continued. With all the hinge metal intension, maximum stretching, and maximum extrusion are accomplished; andthe entirety of the hinge is displaced from between the core end faces13a and 13b. A perfect closure is thus obtained (FIG. 11a). All of thehinge material magnetically damaged by bending is thus out of the finalgap, and will be ground away. A precise parallel faced gap is assured.

Description of a Laminated Core Embodiment FIGS. 12 and 13 show amodification using an assembly of stamped laminations 54 to make up thecore body 55, which is in the stage of construction corresponding tothat represented in FIG. 2 of the earlier embodiment.

The laminations are glued face to face to form a core body such as shownin FIG. 12, capable of being bent as an assembled body. The core form,made up of these assembled laminations, has an apex 56 at the top ofconvergent legs or segments 57, from which extend horizontal core sides58 and erect end legs 59. The top surface of the apex and the topsurfaces of the legs 59 are simultaneously lap ground to a preciselyfiat, common level, as in the earlier embodiments. Coils 60 are wound onthe core sides 58. As before, the grinding of the apex takes the corematerial down to a thickness of the order of a few mils at center. Ahinge is thus provided as in the earlier embodiment, and the core isbent on this hinge to the form illustrated in FIG. 13, and the hingelater ground away.

Selected numbers of the laminations may be wrapped with coils inindividual bundles, which may be used sin gly or in groups for single ormulti-track heads. The core is thus assembled from laminations stampedinitially into the shape of FIG. 12, rather than bent initially fromcore ribbon material.

Description of a Further Embodiment Reference is next directed to FIG.14, showing another embodiment of the invention. This embodiment isshown as using a lamination type core, in general respects like that ofFIGS. 12 and 13; but in this case, the apex of the W-form is comprisedpartly of core metal and partly of aformation on a plastic or metal corehousing or holder, conveniently a die casting. The hinge on which thetwo core halves are bent is actually a part of the apex formation of thehousing member.

The core housing member or holder 66 is shown as a generally rectangularhollow body or box, open on one side,'and composed of a material, metalor plastic, which is relatively ductile and bendable, without springback, when reduced to a thickness of the order of a few mils, and willthus have the characteristics to serve as a hinge of the type describedherein above.

The core, composed of laminations of the same general character as thoseof FIGS. 12 and 13, excepting for a discontinuity at the center, hasangularly related, upwardly convergent segments 67, sides 68, and endlegs 69, much as in FIG. 12. The convergent segments 67 are fabricatedto comprise outer legs 67a of core metal, integral with the side legs68, and inner legs 67b integral with and rising from the housing bottom,and lying contiguously with the outer legs 67a. Segments 67 are thusformed with a compound apex 74. The housing, the end legs 69, and theapex 74 are constructed to rise initially to the level indicated by thephantom lines, and are then simultaneously ground down to the levelshown in full lines in FIG. 14. The inside surfaces of the angularconvergent housing legs 67b merge into a concave curveor fillet 73. Thelap grinding step grinds the apex, made up of the coplanar uppermostsurfaces of the legs 67a and 67b, precisely flat. The ground upper endsof the legs 67a thus have a gap or discontinuity precisely filled by theground upper ends of the legs 67b. The lap grinding is continued untilthe ground apex region at the top of the legs is reduced to a slightthickness, so as to give the hinge formed thereby the hinge bendingquality desired, which may vary with dimensions as well as the materialof the housing. The formations described provide the housing with a deepindentation rising from the bottom to concavity 73, thus adapting thehousing to bend on the hinge. Coils 75 are wound in the side legs 68.

The housing and the two core metal parts are then bent through 180 onthe hinge, turning about an axis just above the underside of the arch atthe top of the two housing legs 67b. The core and housing are then inthe position shown in FIG. 15, a gap filler having been inserted betweenthe two closing core ends, as before. The holder is then filled inaround the core with potting material 90. The extruded hinge loop 80 andportions of the housing thereadjacent are then ground away on the convexgrind line 91, affording a finished convex curvature for the gap end ofthe housing and also for the core tips adjacent the gap.

The present process for making a core is of course subject to variationwithin the scope of the invention, and of course, the design of the corecan be modified in various ways to accommodate it to the process, or toenhance its own characteristics, all without departing from the scope ofthe invention.

What is claimed is: 1. The process of making a core for theelectromagnet of a magnetic recorder or playback head, that comprises:

forming a strip of core material to have two angularly related centralsegments converging to an apex, and to have two portions extendinggenerally oppositely to. one another from the ends of said segmentsremote from said apex, with said last mentioned portions merging intoends which extend toward and substantially to a reference planeperpendicular to a center plane disposed between said angularly relatedsegments and bisecting said apex, said reference plane contacting saidapex; lap grinding said core ends and said apex to parallel surfaces ofprecise flatness, until the remaining core metal of the apex is of onlyslight thickness at said center plane, so as to provide a thin,bendable, core metal hinge, I

bending the core form so made through an angle of 180 on said hingeuntil the ground ends of said strip abut one another to form a closedback gap, and so that the portions of the precisely ground flat surfaceon opposite sides of said hinge come into substantial face to faceopposition, with the metal of the hinge rising substantially out of thegap to form an exterior ridge owing to said 180 bending of said form,and

grinding away said hinge ridge and a portion of the core metalthereunder to form a final head surface configuration having a recordingand/or playback gap therein.

2. The process according to claim 1, including lap grinding said coreends and said apex simultaneously with a single lap to achieve'lappedsurfaces which are both precisely flat and precisely rectilinearlyaligned.

3. The process according to claim 1, including assembling the originalstrip of core material from a plurality of stamped out laminations.

4. The process of claim 1 including, following closing of the back gap,introducing the resulting assembly into a case having an apertured headwall, so that the hinge ridge projects therethrough,

filling the case with a hard setting potting compound that, when set,will preserve dimensional accuracy of and spacial relationships ofparts, and

in the final grinding operation, grinding the apertured head wall to thesame contour.

5. The process according to claim 1, including further forming atapered, wedge-shaped indentation transversely across the center of theflat ground surface of the apex to promote extrusion of the hingeforming metal of the core from between the two hinging halves of theflat ground surface of the apex.

6. The subject matter of claim 1, including also inserting anon-magnetic gap filler strip in the pocket between the two hinginghalves of the flat ground surface of the apex.

7. The process of making a core for the electromagnet of a magneticrecorder or playback head, that comprises:

bending a flat strip-of magnetic core metal to form twoangularly'related central strip segments converging to an apex; bendingthe strip at the ends of said segments remote from said apex to form twostrip portions which extend generally oppositely from the segment endsand'then toward a reference plane perpendicular to a center planedisposed between said angularly related segments and bisecting saidapex, said refer- I ence plane contacting said apex;

grinding said 'apex' down to a precisely Hat and horizontal apex surfaceparallel to said reference plane, and substantially bisected by saidcenter plane, leaving a very slight remaining thickness of metal at theintersection of said center plane with the remainder of the apex, andthereby affording a thin, easily bendable core metal hinge,

grinding the terminal ends of said strip portions to precise flatnessand at locations and angles which will cause them to precisely abut whenthe core strip is bent through on said bendable hinge,

bending the intermediate core form so made through an angle of 180 onsaid hinge until the ground ends of said strip are in such an abuttingrelationship, and so that the portions of the precisely ground flatsurface on opposite sides of said hinge come. into substantial face toface opposition, with the metal of the hinge rising substantially out ofthe gap to form an exterior ridge owing to said 180 bending of saidform, and v inserting and pinching a non-magnetic gap filler shimbetween said surface portions of said gap during the immediatelypreceding step.

8. The process according to claim 7, wherein the strip is so bent thatthe apical juction between the convergent segments has a convex exteriorcurvature.

9. The process of claim 7, including assembling an electromagnet coilwould on a bobbin onto said strip, over one end thereof, before saidends are closed to form said back gap.

10. The process of claim 4 including, following closing of the back gap,introducing the resulting assembly into a case having an apertured headwall, so that the hinge ridge projects therethrough,

filling the case with a hard setting potting compound that, when set,will preserve dimensional accuracy of and spacial relationships ofparts, and

in the final grinding operation, grinding the apertured head wall to thesame contour.

11. The process according to claim 7, including:

also bending the convergent strip segments away from one another to formthe core with two longitudinally aligned segments which extendoppositely from one another, and

bending the last mentioned segments through 90 i into end segmentsterminating in said ends of said strip.

12. The process according to claim 1, including:

lap grinding said strip ends and said apex simultaneously to a preciselycommon level.

13. The process according to claim 2, including:

forming the underside of the apical junction of the convergent segmentswith a concave contour.

14. The process of making a core for the electromagnet of a magneticrecorder or playback head, that comprises as steps:

providing an elongated core form having two angularly related segmentsconverging to an apex, said segments and apex comprised at least in partof core metal, and having two core metal side segments extendingoppositely from the ends of said angularly related segments remote fromsaid apex, and thence extending toward, and with its ends in proximityto, a reference plane which is perpendicular to a'center plane betweensaidangularly related segments and substantially bisects said apex, saidreference plane lying in relatively close proximity to said apex;

lap grinding said core metal ends and said apex to provide parallelground top surfaces of precise flatness, and until the remainder of theapex is of only slight thickness at said center plane, so as to providea thin, interconnecting hinge on which bending of said angular segmentscan occur;

bending the two angular segments of said core form so made on saidinterconnecting hinge through an angle of so that the ground ends ofsaid form abut one another to form a closed back gap, and so that theportions of the precisely ground flat sur face portions on oppositesides of said hinge come into substantial face to face opposition, witha front gap therebetween, and with the material of the hinge displacedsubstantially out of the gap and looped to form an exterior hinge ridge;

inserting a non-magnetic gap filler into said front gap during saidbending step;

embedding at least portions of said hinge-connected angular segments,after being bent through said 180, in hardening potting compound, and

grinding away said hinge ridge along a smooth surface contour whichexposes said front gap and gap filler.

15. The process according to claim 14, including:

tightly fixing the core form, prior to bending on its hinge, inside anopen-topped housing, with its apex and core form ends facingupward,fabricating said angularly related segments each to comprise an outsideleg integrally joined to the corresponding side segment, and an insideleg contiguous to the corresponding outside leg and integrally joined tosaid housing, forming the upper ends of said outside and inside legs toconstitute the lap ground top surface of the apex, with the lap groundsurface on the inside legs in the middle, and the lap ground surfaces onthe outside legs contiguous thereto and on the outside thereof, formingin the bottom of the housing a deep indentation defined by the innersurfaces of said inside legs and completed at the top by a concavesurface that is only slightly spaced from the ground top surface on saidinside legs, whereby to form said hinge integrally with the housing;

all in such manner that the angular core segments are bent on said hingeby closing said housing parts.

16. The process according to claim 1, including preparing said strip ofcore material by assembling it from a plurality of adhesively joinedlaminations.

1. The process of making a core for the electromagnet of a magneticrecorder or playback head, that comprises: forming a strip of corematerial to have two angularly related central segments converging to anapex, and to have two portions extending generally oppositely to oneanother from the ends of said segments remote from said apex, with saidlast mentioned portions merging into ends which extend toward andsubstantially to a reference plane perpendicular to a center planedisposed between said angularly related segments and bisecting saidapex, said reference plane contacting said apex; lap grinding said coreends and said apex to parallel surfaces of precise flatness, until theremaining core metal of the apex is of only slight thickness at saidcenter plane, so as to provide a thin, bendable, core metal hinge,bending the core form so made through an angle of 180* on said hingeuntil the ground ends of said strip abut one another to form a closedback gap, and so that the portions of the precisely ground flat surfaceon opposite sides of said hinge come into substantial face to faceopposition, with the metal of the hinge rising substantially out of thegap to form an exterior ridge owing to said 180* bending of said form,and grinding away said hinge ridge and a portion of the core metalthereunder to form a final head surface configuration having a recordingand/or playback gap therein.
 2. The process according to claim 1,including lap grinding said core ends and said apex simultaneously witha single lap to achieve lapped surfaces which are both precisely flatand precisely rectilinearly aligned.
 3. The process according to claim1, including assembling the original strip of core material from aplurality of stamped out laminations.
 4. The process of claim 1including, following closing of the back gap, introducing the resultingassembly into a case having an apertured head wall, so that the hingeridge projects therethrough, filling the case with a hard settingpotting compound that, when set, will preserve dimensional accuracy ofand spacial relationships of parts, and in the final grinding operation,grinding the apertured head wall to the same contour.
 5. The processaccording to claim 1, including further forming a tapered, wedge-shapedindentation transversely across the center of the flat ground surface ofthe apex to promote extrusion of the hinge forming metal of the corefrom between the two hinging halves of the flat ground surface of theapex.
 6. The subject matter of claim 1, including also inserting anon-magnetic gap filler strip in the pocket between the two hinginghalves of the flat ground surface of the apex.
 7. The process of makinga core for the electromagnet of a magnetic recorder or playback head,that comprises: bending a flat strip of magnetic core metal to form twoangularly related central strip segments converging to an apex; bendingthe strip at the ends of said segments remote from said apex to form twostrip portions which extend generally oppositely from the segment endsand then toward a reference plane perpendicular to a center planedisposed between said angularly related segments and bisecting saidapex, said reference plane contacting said apex; grinding said apex downto a precisely flat and horizontal apex surface parallel to saidreference plane, and substantially bisected by said center plane,leaving a very slight remaining thickness of metal at the intersectionof said center plane with the remainder of the apex, and therebyaffording a thin, easily bendable core metal hinge, grinding theterminal ends of said strip portions to precise flatness and atlocations and angles which will cause them to precisely abut when thecore strip is bent through 180* on said bendable hinge, bending theintermediate coRe form so made through an angle of 180* on said hingeuntil the ground ends of said strip are in such an abuttingrelationship, and so that the portions of the precisely ground flatsurface on opposite sides of said hinge come into substantial face toface opposition, with the metal of the hinge rising substantially out ofthe gap to form an exterior ridge owing to said 180* bending of saidform, and inserting and pinching a non-magnetic gap filler shim betweensaid surface portions of said gap during the immediately preceding step.8. The process according to claim 7, wherein the strip is so bent thatthe apical juction between the convergent segments has a convex exteriorcurvature.
 9. The process of claim 7, including assembling anelectromagnet coil would on a bobbin onto said strip, over one endthereof, before said ends are closed to form said back gap.
 10. Theprocess of claim 4 including, following closing of the back gap,introducing the resulting assembly into a case having an apertured headwall, so that the hinge ridge projects therethrough, filling the casewith a hard setting potting compound that, when set, will preservedimensional accuracy of and spacial relationships of parts, and in thefinal grinding operation, grinding the apertured head wall to the samecontour.
 11. The process according to claim 7, including: also bendingthe convergent strip segments away from one another to form the corewith two longitudinally aligned segments which extend oppositely fromone another, and bending the last mentioned segments through 90* intoend segments terminating in said ends of said strip.
 12. The processaccording to claim 1, including: lap grinding said strip ends and saidapex simultaneously to a precisely common level.
 13. The processaccording to claim 2, including: forming the underside of the apicaljunction of the convergent segments with a concave contour.
 14. Theprocess of making a core for the electromagnet of a magnetic recorder orplayback head, that comprises as steps: providing an elongated core formhaving two angularly related segments converging to an apex, saidsegments and apex comprised at least in part of core metal, and havingtwo core metal side segments extending oppositely from the ends of saidangularly related segments remote from said apex, and thence extendingtoward, and with its ends in proximity to, a reference plane which isperpendicular to a center plane between said angularly related segmentsand substantially bisects said apex, said reference plane lying inrelatively close proximity to said apex; lap grinding said core metalends and said apex to provide parallel ground top surfaces of preciseflatness, and until the remainder of the apex is of only slightthickness at said center plane, so as to provide a thin, interconnectinghinge on which bending of said angular segments can occur; bending thetwo angular segments of said core form so made on said interconnectinghinge through an angle of 180* so that the ground ends of said form abutone another to form a closed back gap, and so that the portions of theprecisely ground flat surface portions on opposite sides of said hingecome into substantial face to face opposition, with a front gaptherebetween, and with the material of the hinge displaced substantiallyout of the gap and looped to form an exterior hinge ridge; inserting anon-magnetic gap filler into said front gap during said bending step;embedding at least portions of said hinge-connected angular segments,after being bent through said 180*, in hardening potting compound, andgrinding away said hinge ridge along a smooth surface contour whichexposes said front gap and gap filler.
 15. The process according toclaim 14, including: tightly fixing the core form, prior to bending onits hinge, inside an open-topped housing, with its apex and core formends facing upward, fabricating said angularly related segments each tocomprise an outside leg integrally joined to the corresponding sidesegment, and an inside leg contiguous to the corresponding outside legand integrally joined to said housing, forming the upper ends of saidoutside and inside legs to constitute the lap ground top surface of theapex, with the lap ground surface on the inside legs in the middle, andthe lap ground surfaces on the outside legs contiguous thereto and onthe outside thereof, forming in the bottom of the housing a deepindentation defined by the inner surfaces of said inside legs andcompleted at the top by a concave surface that is only slightly spacedfrom the ground top surface on said inside legs, whereby to form saidhinge integrally with the housing; all in such manner that the angularcore segments are bent on said hinge by closing said housing parts. 16.The process according to claim 1, including preparing said strip of corematerial by assembling it from a plurality of adhesively joinedlaminations.